Clang: Process distinct documents concurrently

Speed ups the typical use cases that can profit from this:
 * Change a header file and switch then to source file
 * Open documents one after the other (Follow Symbol)
 * Change documents visible in splits (e.g. by refactoring action)
 * Restore a session with multiple splits

Fixes the test ClangIpcServer.GetCodeCompletionDependingOnArgumets.

Change-Id: Ia575bd59780df14146dfc091a4d48794e4a0543d
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

tests/unit/unittest/sourcerangetest.cpp

@@ -106,8 +106,8 @@ struct Data {
                                     Utf8StringVector(),
                                     translationUnits};
     TranslationUnitCore translationUnitCore{filePath,
-                                            translationUnit.index(),
-                                            translationUnit.cxTranslationUnit()};
+                                            translationUnit.translationUnitCore().cxIndex(),
+                                            translationUnit.translationUnitCore().cxTranslationUnit()};
 
     std::unique_ptr<SourceRangeData> d;
 };